Combine the onions, garlic, and vinegar in a small saucepan. Cook on medium, stirring frequently, for 5-7 minutes or until most of the vinegar is reduced into a thick syrup and has been absorbed by the green onions. Remove from heat and set aside.

Bring a large pot of salted water to a boil. While the water is coming to a boil, combine the vinegar/onion mixture and the butter. Set aside.

When the water begins to boil, add the green beans. Cook for 3-5 minutes or until the beans float to the top and are bright green. Drain. Toss in the balsamic butter and serve immediately. Serves 8.